1. Field of the Invention
The present invention relates in general to illumination systems for photographic printers, and more particularly to such illuminations systems adapted for suppressing the appearance of film scratches in the print.
2. Description Related to the Problem
FIG. 1 is a schematic diagram of a prior art color photographic printer of the type commonly employed by photofinishers for making color photographic prints from color negatives. The printer, generally designated 20, includes a printing lens 22, having an aperture stop 24, for projecting an image on a film 26, centered in a film gate 28, onto photographic paper 30. The film 26 is transported through the printer by a film transport system including a takeup 32 and a supply 34. Similarly, the photographic paper is transported through the printer by a paper transport system including a paper takeup 36 and paper supply 38. An illumination system for illuminating the portion of film 26 in film gate 28 includes a lamphouse generally designated 40, a set of dichroic filters 42 for adjusting the color balance of light emitted from the lamphouse 40 and a "tapered integrating bar" 44 generally of rectangular cross section and having a slight taper .alpha. of about 0.degree.-2.degree., depending on the film format. The integrating bar gathers some of the off-axis illumination emitted by the lamphouse 40 and redirects the light into the film gate 28. Lamp house 40 comprises a high intensity light bulb 46, an ellipsoidal reflector 48 and a cold mirror 50. Most of the light from lamp 46 is directed by ellipsoidal reflector 48 toward cold mirror 50. The infrared portion of the spectrum passes through cold mirror 50 and the visible portion of the spectrum, required for printing, is reflected from cold mirror 50 back through a hole 52 in ellipsoidal reflector 48 toward film gate 28.
The three main goals in the design of an illumination system for a photographic printer are uniformity, scratch suppression and high efficiency. Uniformity means that the illumination at the paper plane is substantially uniform in both intensity and color balance so that there are not objectionably visible "hot spots" or colored patterns in the resulting print. The role of the illumination system in suppressing scratches can be explained with reference to FIG. 2. A cross section of a scratch in film 26 is schematically depicted in FIG. 2 as a V-shaped groove having sides or facets 54 and 56. Each facet of a scratch causes the film to act as a prism to refract a ray of light away from its normal path. As shown in FIG. 2, facet 54 of the scratch refracts an axial ray 58, that normally would have passed through aperture 24 of lens 22, out of the aperture. If all of the light rays from the illumination system in the vicinity of the scratch were parallel to ray 58, i.e. if it is a specular light source, no light in the region of facet 54 would reach the paper, thereby causing the scratch to appear as a white line in the resulting print when printing from a negative. However, if some off-axis rays are available, such as ray 60 which normally would not have passed through aperture 24, these off-axis rays will be refracted by facet 54 into the aperture 24 of lens 22, thereby "filling in" the light that was lost and suppressing the appearance of the scratch in the print. Thus, an illumination system which provides highly diffuse light, in which many off-axis rays are available, is desirable for suppressing the appearance of scratches in negatives.
The cost of providing a source of highly diffuse light for good scratch suppression is a loss in efficiency. The efficiency of an illumination system for a photographic printer is a measure of how much light coming from the illumination system actually gets through the aperture of the printing lens. When the light is highly diffuse, necessarily much of the light emitted by the illumination system will miss the aperture 24. The more light that can be passed through the aperture to the printing lens, the shorter the exposures need to be to expose a print. In modern high-speed photographic printers, the required exposure time is a critical factor in limiting the top speed of operation of the printer. Of course, the amount of light that passes through the printing lens aperture can be increased by increasing the intensity of the light source without regard to the efficiency of the illumination system. However, there is a limit imposed by power and cooling requirements that cannot be exceeded. Most modern printer light sources already operate at or near this limit. Therefore, increasing the efficiency of an illuminating system is one method of improving the maximum possible speed of the printer.
The prior art illumination system described thus far with reference to FIG. 1 would produce a pattern of illumination as shown in FIG. 3. A cross section of the illumination at the output end of the integrating bar 44 describes a fan-like pattern 62 composed of a plurality of beams, for example a-e. As shown in FIG. 3, the beam emerging from the integrating bar has an angular spread of about .+-.50.degree.. The zeroth ordered beam "c" is produced by the rays of light which pass through integrating bar 44 with no internal reflections. The first order beams "b" and "d" are formed in rays of light passing through the integrating bar 44 with only one reflection. The second order beams "a" and "e" are formed by rays of light passing through the integrating bar with two reflections. As can be seen in FIG. 3, the illumination system is capable of providing good scratch suppression since there are plenty of off-axis rays of light that can be shifted into the printing lens 22 by the facets by scratches on the film. Each facet of a scratch may be imagined as having the effect of pivoting the fan of light rays about the end of integrating bar 44. Unfortunately, however, if the illuminating system shown in FIG. 3, is used "as is" the illumination in the paper plane 30 exhibits a pattern of "hot spots" 64, shown in FIG. 4. The reason for this nonuniformity in illumination can be seen in FIG. 3. The illumination from the zeroth order beam "c" is focused by lens 22 at some point between the printing lens and the paper plane 30 and forms a distribution of illumination 66 near the center portion of the paper plane 30. The light from first order beams "b" and "d" are likewise focused by lens 22 in a plane between the lens and the paper plane 30 and form distributions of illumination 68 and 70 to either side of the central distribution of illumination 66.
To achieve uniformity in the prior art printer illumination system, a strong volume diffuser such as a sheet of translucent Plexiglas 72 (see FIG. 5) was placed at the end of the integrating bar 44. The resulting pattern of light emerging from the end of the bar is shown in FIG. 5. As seen in FIG. 5, uniformity of the illumination is greatly improved without sacrificing the scratch suppressing qualities of the illumination system. However, since the angular spread of the fan of light 74 emerging from the illumination system has been greatly increased, from about .+-.50.degree. to .+-.90.degree. with an accompanying decrease in total transmittance, the effieciency of the illumination system has been decreased by about half. Much more of the light emitted by the illumination system now escapes the aperture of printing lens 22. A tradeoff has been effected between uniformity and efficiency. As stated above, this reduction in efficiency lowers the maximum speed at which the printer can be operated. Thus the inventor was faced with the problem of providing an illumination system for a photographic printer that would preserve the scratch suppressing characteristics and high uniformity of the prior art illumination systems, while increasing the illumination efficiency thereof.